Not working well on Windows

[provegard]

It seems to assume that paths use forward slash as file separator. This is problematic in at least two places:

1. When passing an absolute path to require.
2. When passing a relative path to require from within a required module, and the path is resolved against the parent path.

1 is solvable by adding a require wrapper that rewrites a Windows path to Unix form. 2 is harder, since the require function in a required module cannot be wrapped if I'm reading the code correctly.

[lance]

@provegard This PR should have resolved your windows issues. #19

Please let me know, if not.